The present invention relates to a sheet feeding device for an impact-type printer with a platen. More particularly, the present invention relates to means for causing pressure rollers of the sheet feeding device to be displaced into and out of pressing engagement with the platen of an impact-type printer.
A conventional sheet feeding device in an impact-type printer is shown in FIG. 1. As can be seen, impact-type printers conventionally include a platen 1 and a paper guide 2 for guiding a printing sheet A along the cylindrical exterior surface of the platen 1. A crank arm 3 is positioned below the paper guide 2 in such a manner that it is pivotal about a shaft 4. The crank arm 3 is in slidable contact with a cam roller 6 which is positioned at one end of a lever 5, the latter being pivotal abaout shaft 5a. Therefore, as the lever 5 pivots about shaft 5a (as indicated by the arrow 5b in FIG. 1), the crank arm 3 responsively pivots about shaft 4 as shown by arrow 5c in FIG. 1. In such a manner, a pressure roller 7 which is provided at one end of the crank arm 3 is caused to either press against or be displaced from the sheet A on the platen 1 through an opening 8 formed in the paper guide 2 in dependence upon the direction of pivotal movement of lever 5 about shaft 5a.
The conventional sheet feeding mechanism described above is disadvantageous in several respects. For example, separate mechanisms are required in order to operate the pressure roller 7 via the crank arm 3, the lever 5 and the cam roller 6. As the pressure roller 7 is operated through a plurality of components, the pressure roller operating structure is disadvantageously large in the number of components, intricate in construction and high in manufacturing cost. In general, the lever 5 and the cam roller 6 are laterally provided relative to the platen 1 so that they do not obstruct the operations of the other components comprising the printer. Therefore, when the pressure roller 7 is urged against the platen 1, the pressure may not be uniform over longitudinal length of the platen.